Preparation and Microcellular Foaming Using Supercritical Fluid of PP Matrix Composites

장진수 경상대학교 대학원 2010 국내박사

고분자 산업이 발전하기 위한 동기로써 고분자 내의 다양한 가스기포를 도입함으로써 고분자의 함량을 줄이는 것과 재생된 재료를 사용함으로써 가격을 낮추는 것이다. 본 논문의 부분에서는 이산화탄소발포제를 이용한 일괄 발포기술로 폴리프로필렌과 폐타이어 분말의 복합 발포체를 제조하였다. 퍠타이어 분말의 함량과 산무수물이 도입된 폴리프로필렌의 결정화는 점도와 셀의 형태를 통해 연구하였다. 폴리프로필렌/폐타이어분말의 복합 발포체는 다양한 기포구조형태를 보였고, 폐타이어 분말이 관찰 된 부분의 셀 구조가 크게 나타났으며 이러한 폐타이어 분말의 집중은 셀 구조를 약하게 만들어 폴리프로필렌과 폐타이어 분말의 접착력을 저하시키기 때문에 포화 압력과 온도와 같은 제조과정 조건들이 셀 구조에 어떠한 영향을 미치는가에 대하여 연구하였다. 산무수물이 도입된 폴리프로필렌과 폐타이어분말의 함량이 50:50 비율의 복합체 미세발포 셀은 매우 균일하고 고른 구조를 보였는데 이는 낮은 밀도와 기계적 물성향상과 관련되어 있다. 또한 기계적 특성의 평가 결과 발포 조건들은 셀 형태에 중요한 영향을 미쳤다. 폐타이어 분말의 재활용 요구들은 특별한 기술들이 필요로 하는데 이는 폐타이어 고무는 열경화성 수지이며 열가소성 수지와 같이 재활용이 되지 않기 때문이다. 폐타이어 분말 재활용에 대한 가능성은 열가소성수지와의 결합을 통한 TPVs 를 통해 가능해졌다. 본 논문의 부분에서는, Bituman 처리된 폐타이어 분말을 근거로 뛰어난 TPVs 개발이다. Bituman과 폐타이어분말의 함량의 영향, 산무수물이 도입 된 SEBS-g-MA 의 함량이 TPVs 소재의 마지막 기계적 물성에 주는 영향을 시행착오와 오차를 줄일 수 있는 실험디자인(DOE)으로 접근하였다. 최적화는 하이브리드 인공 신경망 - 유전자 알고리즘 hybrid artificial neural network-genetic algorithm(ANN-GA) 기술을 이용하여 마무리하였다. 양적 관계는 소재의 함량과 기계적 물성의 윤곽선이 같은 형식으로 나타났으며, 이는 실험적으로 최적의 함량인 것이 확인되었다. 또한 폴리프로필렌/폐타이어 분말 복합 발포체는 이산화탄소 발포제를 이용한 일괄 발포기술로 제조하였으며 Bituman과 SEBS-g-MA 함량, 압력 그리고 온도 조건이 마지막 셀 구조에 미치는 영향에 대해 연구하였다. 목분이 첨가 된 플라스틱 복합체(WPC)의 주요한 결점은 나무와 플라스틱의 높은 밀도를 비교하였을 때 근본적으로 모든 고분자수지와의 낮은 결합력이다. 높은 물성과 경량성의 WPC 소재 개발을 위해서는 압출기의 스크류 조합과 속도 그리고 충진재인 실리카의 함량, 다양한 상용화제를 이용한WPC복합체의 물리-기계적 특성과 발포 특성에 대한 연구가 이루어져야 한다. 그래서 본 실험에서는 폴리프로필렌과/목분 섬유 복합체를 동방향 이축 스크류 압출기를 통해 제조하였다. 미세발포 구조에 가까운WPC 복합 발포체는 pressure-quench batch 공정법을 통해 제조되었다, 우선 압출 최적 조건 결정을 위해 스크류 조합과 속도에 대한 실험을 진행하였다. 다음으로 최적조건의 가공공정 하에서 실리카 함량의 영향을 WPC/silica 복합체 물성을 통해 연구하였고, 복합체의 밀도와 관련된 마지막 셀 구조에 대해서 연구하였다. 마지막으로 다양한 상용화제의 영향을 기계적, 형태학적, 결정성, 유변학적 특성을 평가를 통해 알아보고, 마지막 셀 구조와 관련된 밀도에 관해 조사하였다. 또 다른 WPC의 결점으로 그들은 높은 인화성을 꼽을 수 있다. 유기소재의 경우, 고분자와 나무 섬유는 불에 매우 민감하며, 복합 소재의 난연성을 향상은 나무 섬유 복합체 제품의 안정성 요구에 없어서는 안될 매우 중요한 항목이다. 본 논문의 부분에서는, 암모니움폴리포스플레이트(APP)와 실리카를 난연제로 이용하였고, 이렇게 제조 된 복합체의 분해특성과 난연성, 발포성 그리고 기계적 물성에 대해 연구하였다. 산소 제한 지수 (LOI)는 콘 칼로리메터와 열중량분석기 (TGA)를 통해 난연특성을 연구하였다. 이와 함께, WPC 복합 발포체는 이산화탄소 발포제를 이용한 일괄 발포기술로 제조 하였으며, 마지막 셀 구조에 미치는 APP와 실리카 함량 그리고 가공 조건에 대해 토의하였다. Motivation of the polymer industry towards to introduction of a large number of gas bubbles inside a polymer was to reduce the amount of polymer, another way to drive down costs is to use recycled materials. In this dissertation, PP/WGRT composite foams were produced with the batch foaming technique using CO₂ as blowing agent. The content of waste ground rubber tire (WGRT) and maleic anhydride grafted PP on the crystallinity, viscosity and cell morphology were studied. The foam of PP/WGRT composites shows a unique bimodal (large and small) cellular structure, in which the large-cells embrace a WGRT powder, the cell structure was deteriorated at high concentration of waste rubber because of the poor adhesion of PP and waste rubber, the effect of processing conditions such as saturation pressure and temperature on the cell structures were also investigated. PP-g-MA/WGRT (50/50) produced microcellular foams with a very fine and uniform cell structure, lower relative densities and improved mechanical properties. The results of mechanical properties were significantly affected by the foaming conditions which varied with the cell morphologies. Recycling of waste ground rubber tire requires special techniques because waste ground rubber tire is a thermoset material, which cannot be reprocessed like thermoplastics. A promising way of ‘recycling’ waste ground rubber tire powder (WGRT) is to incorporate it into thermoplastics to obtain thermoplastic vulcanizes (TPVs). In this part, one novel TPV material based on bitumen treated waste ground rubber tire powder (WGRT) was produced. The effect of ratio of bitumen and WGRT, the maleic anhydride-grafted styrene-ethylene-butylene-styrene (SEBS-g-MA) content on the final mechanical properties of TPV were predicted by design of experiments (DOE) rather than by “trial and error” approach. Optimization was done using hybrid artificial neural network-genetic algorithm (ANN-GA) technique. A quantitative relationship was presented between the material concentration and the mechanical properties as a set of contour plots, which were confirmed experimentally by testing the optimum ratio. At the same time, PP/WGRT composite foams were produced with the batch foaming technique using CO₂ as blowing agent. The effect of bitumen content, SEBS-g-MA, pressure and temperature on the final cell structure was investigated. The main drawbacks of Wood-fiber filled plastic composites (WPC) are their poor adhesion to basically all matrix polymers and high density compared to natural wood and certain plastics. In order to produced high properties and lightweight of WPC product, the objective of this part was to investigate the effects of screw configuration, screw speed, silica content and various compatibilizer on the physico-mechanical and foaming properties of Wood-fiber/PP composites. Wood-fiber/PP composites were produced on the intermeshing co-rotating twin screw extruder. Microcellular closed cell Wood-fiber/PP composite foams were prepared using pressure-quench batch process method. First, an attempt has been made to determine the optimum conditions of extrusion that involve are screw configurations, screw speed. And then, under the optimal processing conditions, the effect of silica content on the physico-mechanical properties of Wood-fiber/PP/Silica composite, and on the final cell morphology as well as the relative density of the foamed Wood-fiber/PP/Silica composites were studied. Finally, and various compatibilizer on the mechanical properties, morphology, crystallinity and rheological properties of Wood-fiber/PP composites, and on the final cell morphology as well as the relative density were investigated. Another drawback of WPCs is their high flammability. As organic materials, the polymers and the wood fibers are very sensitive to flame; improvement of flame retardancy of the composite materials has become more and more important in order to comply with the safety requirements of the wood fiber-composite products. In this part, the Ammonium polyphosphate (APP) and silica were used as flame retardants, the mechanical properties, flame retardancy, thermal degradation and foaming properties of Wood-fiber/PP composites have been investigated. The limiting oxygen index (LOI), cone calorimeter and thermal gravimetric analysis (TGA) were employed for the study of fire retardance. The synergistic effect of APP and silica on the flame retardant properties was also studied. Meanwhile, Wood-fiber/PP composite foams were produced with the batch foaming technique using CO₂ as blowing agent. The effect of APP and silica content and processing conditions on the final cell structure was discussed.

탄소-현무암섬유 강화 에폭시 층간 복합재료의 파괴거동 및 하이브리드 효과

이데와그디아리 전북대학교 일반대학원 2013 국내박사

Composite materials have been well documented in engineering technology. They have replaced traditional materials (metals and alloys) in engineering technologies for over two decades due to their performance and characteristics. Composite structure generally consists of reinforcement and matrix. Carbon fiber is a superior in organic fiber utilized as a composite reinforcement. It is widely applied especially on advanced technological purposes due to its inherent properties but it has the drawback of being characteristically brittle and expensive. Hybridization is a good approach to make good the weakness of a carbon fiber wherein it is usually combined with glass fibers. But the use of glass fibers is not worthy due to its toxicity although it improves the characteristics and production costs of the composite. The present study is focused on interply hybrid composite between the carbon fabric and basalt fiber as a function of basalt fabric content and alternate stacking sequence. Tensile, flexural and Mode I fracture toughness tests were conducted to investigate the mechanical characteristics of an interply hybrid composite. The purpose of this study is to describe the mechanical characteristics of a carbon-basalt/epoxy interply hybrid composite and to provide additional information about basalt fabric-based hybrid composites. Specifically, it is to measure and investigate the mechanical characteristics of interply hybrid composites based on the basalt fabrics content and alternate stacking sequence on quasi-statics tests. Tensile stress–strain curves were measured via standardized tests of ASTM D 638 to provide the elastic modulus, yield stress, ultimate strength, strain to failures, as well as a preliminary assessment of toughness. Flexural test was conducted with standardized tests of ASTM D 790 to find out the bending stiffness and strength. The fracture toughness, KIC, is calculated using the compliance method. The fracture toughness was determined as the function of specimen geometry, loading and crack extension of interply hybrid composites in accordance with the standardized ASTM D 5045. The effect of hybridization was analyzed using the Rule of Mixture. Each sample of the interply hybrid composite used in this study was fabricated using the vacuum assisted resin transfer molding (VARTM) process. Ten plies of carbon and basalt fabrics were prepared to be used only for tensile and flexural tests and then eighty plies more for Mode I fracture toughness test. Carbon fabrics C120-3K and Basalt fabrics EcoB4-F210 were employed respectively and furthermore, Epoxy resin HTC-667C with modified aliphatic hardener for matrix was used. The structure of the interply hybrid composite is based on its basalt fabric content which varies from 10% to 50% of the carbon fiber reinforced polymer (CFRP) (i.e., C5B¬1C4, C4B2C4, C4B3C3, C3B4C3 and C3B5C2). Furthermore, the structure design was based on the alternate stacking sequence (C3B4C3, B2C6B2 and C2B2C2B2C2). The compact tension (CT) specimen was designed according to basalt content of 10% to 40% of the carbon fabrics reinforced polymer and was based on alternate stacking sequence. All of the samples were cut using a water-jet machine. Five specimens were prepared and examined for each of the test conditions which are assumed anisotropic homogeneous in the two dimensional approach. This study showed that the tensile stress-strain curves presented linearity with increasing basalt fabric contents into the CFRP. as results, we can see that the initial slope of the stress-strain curves demonstrated a proportional decrease. However, the fracture strain showed an increasing trend. The tensile strength for interply hybrid composite as against the CFRP showed a decrease of 9.05%, 14.12%, 23.12%, 28.17% and 36.85% for each increasing basalt fabric content. The Young’s modulus is also decreased by 10.17%, 18.18%, 27.45%, 32.29% and 62.5% for each addition of basalt fabrics, respectively. Moreover, alternate stacking sequence showed that B2C6B2 (D2) has the highest tensile strength and Young's modulus of about 571 MPa and 49.5 GPa (6.53% and 5.32%) compared with C3B4C3 (C4) and C2B2C2B2C2 (E), respectively. This result proved that stacking sequence with basalt fabrics on the exterior region has a major effect on the tensile strength and Young’s modulus but only a slight influence on the tensile strain. The typical load-displacement curve for interply hybrid composite as a function of basalt content showed linear characteristics based on the result of the flexural tests but the stiffness and strength values are less than those of CFRP. In this case, the flexural strength and flexural modulus of interply hybrid composite decreased linearly with increasing basalt fabrics content (10% until 50%) to a maximum of about 18.66% and 17.5% as against the CFRP, respectively. In the flexural experiment as a function of arrangement of sequences between carbon fabric and basalt fabric (4 ply basalt fabrics and 6 ply carbon fabrics), the initial slopes of interply hybrid composites exhibited linear characteristics. Consequently, the flexural strength and flexural modulus of elasticity have high values when the position of carbon fabrics at the compressive side compared with hybrid composite with basalt fabric at the compressive side. Specifically, the interply hybrid composite with arrangement sequence of C2B2C2B2C2 has a high modulus of about 46.408 GPa. The present results suggest that the incorporation of basalt fabrics into CFRP could improve the mechanical characteristic depending on the stacking sequences. Mode I fracture toughness test showed that the fracture toughness KIC of interply hybrid composites decreases with increasing basalt fabric content in the CFRP. The interply hybrid composite (C36B8C36) decreased by 3.15% as against the CFRP. Additionally, increasing basalt fabric content (10% until 40%) of the CFRP decreased the fracture toughness by 5.46%, 8.87% and 7.56% for each addition of basalt fabrics, respectively. The interply hybrid composite as a function of alternate stacking sequence between the carbon and basalt fabrics have shown an increase in fracture toughness value. Results showed that the interply hybrid composite (C16B16C16B16C16) has the highest fracture toughness value of about 36.083 (MPa√m) compared with that of C24B32C24 and B16C64B16 proving that the arrangement of fabric between carbon and basalt fabrics significantly influences the fracture toughness of composite materials. Fracture surface analysis through SEM showed that pull-out, de-bonding, fiber unraveled and delamination were common occurring features on interply hybrid composite of carbon fabric and basalt fabric epoxy matrix. In addition, shear hackles are formed due to the matrix (epoxy) cracks on the plane of maximum tension and perpendicular to the direction of delamination. Carbon fabrics generally fail in brittle manner while basalt fabrics fail in an unraveled manner. To sum up the hybridization effect based on the rule of mixtures, it can be described that increasing basalt fiber content and the use of alternate stacking sequence influenced the mechanical characteristics of composite materials. Hybridization also gives a good balance between the desired mechanical properties and production costs.

산화아연 구조 및 하이브리드 네트워크 제어를 통한 복합재료 기반 압전 나노발전기의 성능 향상

김건수 전북대학교 일반대학원 2023 국내석사

Composite-based piezoelectric nanogenerator (PNG) is flexible and can withstand greater external force, so it can be used in various fields. The aspect ratio (AR) of zinc oxide (ZnO), which is mainly used as a piezoelectric filler in PNG with a simple and environmentally friendly synthesizing method, can have a significant impact on the output performance of the nanogenerator. However, the relationship between the AR of hydrothermal grown ZnO filler and the piezoelectric performance of the composite PNG has not been clearly identified. In addition, in order to miniaturize of piezoelectric device, and use it anywhere and anytime, it is required to develop a PNG with improved energy harvesting performance, which responds to various stimuli other than the external forces. In this study, the effects of ZnO morphology and AR with the processing conditions of hydrothermal synthesis on piezoelectric performances of composite-based PNG were systematically analyzed. The ZnO AR increased from 3.3 to 26.3 as the hydrothermal growth temperature increased from 65 ℃ to 95 ℃ at the optimum growth time of 60 min. The piezoelectric current/voltage of the composite-based PNG was improved by 1266/221 and 549/419%, during repetitive mechanical force and hammer stamping, respectively, according to the increase in the ZnO AR. As a result, it was identified that the ZnO AR controlled by the processing conditions of the hydrothermal synthesis had a significant impact on the piezoelectric performance of the ZnO-based composite PNG. Improved piezoelectric performance was investigated through a hybrid structure in which piezoelectric filler was attached and grown on nanocarbon filler. The piezoelectric current of the composite PNG incorporating ZnO@GNP and ZnO@CNT hybrid fillers in which ZnO was attached and grown on nanocarbon fillers was improved by 98 and 176% under hammer stamping, respectively, compared to that of the composite PNG incorporating only ZnO piezoelectric fillers. Therefore, It was identified that the incorporation of the ZnO@nanocarbon hybrid filler to which ZnO was attached and grown significant affected the piezoelectric performance of the composite PNG. In addition, The attached and grown mechanism of ZnO on highly drawn PP fiber and the piezoelectric performance of fabricated ZnO-based PP fabric were investigated according to the pretreatment method in the two-step hydrothermal synthesis method. The attached and grown of ZnO on highly drawn PP fiber substrate was successfully achieved in the pretreatment method using a sonicator. The piezoelectric performance of the fabric fabricated by weaving the ZnO@PP fiber is generated by applying an external force to the piezoelectric material based on the shrinkage force generated when the highly drawn PP fabric was shrunken due to thermal energy, and a piezoelectric current of about 20 nA was generated. These results identified that the synthesis of ZnO attached and grown on organic materials is possible by controlling the pretreatment method in the two-step hydrothermal synthesis process, and confirmed that the piezoelectric phenomenon of new mechanism witch responds to thermal energy.

A Study on anisotropically assembled Ceramic-polymer composites and their Application for high heat spreaders

유명재 고려대학교 대학원 2012 국내박사

The thermal conductive properties of anisotropically assembled ceramic-polymer composites were investigated and their potential for application as high heat spreader device was studied in this work. Various insulating fillers with high thermal conductive property were utilized to fabricate composite materials. Anisotropic assemble of fillers within composite material was achieved by applying electric field. Silicone resin was selected as polymer matrix in order to apply high electric field strength to fabricate composite material. According to filler material, filler morphology, loading amount, applied electric field type and electric field strength a variety of microstructure fabrication was enabled. Alumina filler loaded composites formed anisotropic microstructures with applied DC electric and AC electric field. According to morphology of filler different anisotropic microstructures were fabricated. For 20vol% spherical shape filler loaded composite by applying DC electric field of 1kV/mm resulted in composite with increased thermal conductivity of 0.46W/mK. For plate shaped filler with 20vol% filler loading the thermal conductivity increased up to 0.44W/m K. Also by increasing the applied DC electric field strength the thermal conductivity increased to 0.54W/m K. By applying AC electric field even higher thermal conductivity of 0.71W/m K was obtained which was estimated to be due to aggregation of the fabricated anisotropic microstructures. For 40vol% filler loaded composite, thermal conductivity of 1W/m K was enabled but the thermal conductive property decreased with increasing electric field strength. But by filler loading with bi-modal size distribution the fabricated anisotropic composite retained its thermal conductive property even increasing electric field strength. For boron nitride filler loaded composites no detectable formation of anisotropic microstructure was observed regardless of the type of the applied electric field. However due to the inherent high thermal conductive property of boron nitride filler, the 20vol% filler loaded composite achieved thermal conductivity of 0.75W/ m K. For aluminum nitride filler loaded composite no anisotropic microstructure was fabricated with applying DC electric field. Rather the fillers showed distinct separation from the polymer matrix resulting in reduced thermal conductive properties. But by applying AC electric field, assembly of fillers into anisotropic microstructure was achieved. Thermal conductivity of 0.91W/m K was obtained with 20vol% filler loaded composite. Diamond filler loaded composites achieved various anisotropic microstructures according to applied electric field type and strength. For 20vol% filler loaded composite the highest thermal conductivity of 0.74W/m K was obtained by applying AC electric field. It was determined that by assemble of fillers into anisotropic microstructure by applying electric field, the thermal conductivity of the composite was increased drastically. To acquire a solution for obtaining the optimum electric field condition for fabricating anisotropic microstructure a Rheometer was designed and utilized. From the measured signal values the storage shear modulus of the composite suspension was calculated. Samples were fabricated according to obtained electric field condition and it was found that for the electric field condition with highest storage shear modulus value the composite with highest thermal conductivity was obtained. Finally a high heat spreader device was fabricated with the anisotropically assembled composite material. Compared to device fabricated with conventionally prepared composite material the developed composite material showed significantly lowered thermal resistance of 1.4W/K thus promising to be a good solution for high power devices where thermal heat dissipation is a critical issue for device performance and reliability.

Failure Analysis Of Composite Laminates With Low Velocity Impact

정소량 명지대학교 대학원 2013 국내석사

복합재료적층판재는 상대적으로 우수한 높은 강도, 강성 및 다양한 설계의 특성으로 인하여 건설, 항공 우주, 자동차 및 스포츠 장비 등 각종 분야에 다양하게 사용된다. 충격에 의해 발생한 손상은 유지 보수 및 서비스 작업 중에 발생할 수 있으며 이러한 저속 충격은 심각한 손상을 야기할 수 있다. 이러한 손상들은 시각적으로 발견하기 어렵고 쉘 및 판재 구조물의 강성과 강도의 심각한 감소를 가져올 수 있으며 따라서 복합재료의 내충격성이 매우 약해질 수 있다. 따라서, 저속충격에 의한 복합재료적층판의 파손을 예측하는 연구가 필요하다. 본 논문에서 3가지의 다른 충격하중 (충격에너지) 수준에 따라 저속충격에 의한 복합재료적층판의 충격 손상 영향을 유한 요소법으로 연구한다. 또한 보다 개선된 파손 판별식이 자세히 제시되었다. 실험 시편의 재료는 보잉 747 기체에 사용된 재료와 동일하다. 우선, ANSYS 유한 요소 시뮤레이션의 플랫폼에 기초를 두어 복합 적층판재 (〖[0/45/-45/90]〗_s)의 유한 요소모델을 개발하였다. 이 모델에서는 Chang-Chang의 판별식으로 부터 유도된 섬유 파손, 기지재료 균열, 기지재료 파괴의 판별식을 각각 사용하여 저촉충격 하중시 각 장마다 파손 판별여부를 결정하였다. 또한 복합재료적층판의 파손을 실험적으로 예측하기 위하여 스트레인게이지를 사용 변형률과 응력 계산 결과를 사용하였다. 그결과는 시뮤레이션 결과와 비교하였다. 실험 결과와 시뮤레이션 결과는 대체적으로 일치함을 알 수 있었다. 그리고 복합배료적층판의 각 장의 파손여부를 평가하기 위하여 본 연구를 위해 개발된 유한 요소 모델을 이용하였다. 4개의 파손 기준식을 통하여 섬유강화 복합재료의 각 장에서 파손발생 여부를 알아낼 수 있었다. 증가한 충격하중, 충격속도 와 충격에너지에 따라서 섬유강화 복합재료의 내부 손상이 증가함을 알 수 있었다. 또한 복합재료적층판에 가해진 작은 충격하중에 시각적으로 발견 안되는 파손사이즈와 위치를 본 연구에서 개발한 유한 요소 모델을 사용 식별 할 수 있다. Reinforced composite laminates are used in a wide variety of applications such as construction, aerospace, automobiles and sports equipment due to their high specific strength, high specific modulus and strong designable characteristic. Impact-induced damage may arise during manufacture, maintenance and service operation. Low velocity impact cause significant damage, such damage is very difficult to be detected visually and can result in severe reductions of the shell structures stiffness and strength, besides impact resistance of composite material is a serious drawback. Therefore, it is very necessary to study predict damage failure in reinforced composite laminated structure subjected to low velocity impact. In this paper, the impact damage effects of reinforced composite laminate and shell structures subjected to low velocity impact with three different loading levels were investigated by finite element method, the implementation of improved failure criteria for laminated composite structures is given in detail. The material of experimental specimens is the same as the Boeing 747 fuselage. Firstly, based on the ANSYS finite element simulation platform, the finite element impact model of composite laminate is developed by Shell element with 8 layers by different stacking sequence. In this modeling, mainly consider four failure criteria as fiber breakage, matrix cracking, matrix crushing and delamination of Chang-Chang criteria under low velocity impact loading. In order to validate the finite element model to predict the damage of composite laminate under low velocity impact loading, the simulation results are compared with the experimental results which obtained by strain gages, and it was found that the simulation results were consistent with experimental results, also, the verified finite element model is used to evaluate the damage and failure of each layer of composite laminate. According to the simulation results with four criteria, we can determine whether the damage and failure take place in each layer of reinforced composite. The results show that the internal damage of reinforced composite laminate increase with increasing the impact loading speed and energy. It can be identified with new developed FE model for the damage size and location of the reinforced composite laminate, which we can’t detect visually, subjected to small impact loading.

Interfacial engineering of composite materials via chemical functionalization for mechanical properties control and additive manufacturing

Ding, Ruonan Sungkyunkwan University 2023 국내박사

The development of composite materials is always closely linked with their manufacturing and processing. Since the origination of additive manufacturing (AM also known as 3D printing) in the 1980s, there has been rapid development with increasing interest in AM technologies for composite due to the advantages of high efficiency, resolution, and customization. However, the composite printed layer by layer has great differences in interfacial behaviors, affecting overall performance. Designing suitable interfaces according to different constituents to control the mechanical properties of composites is a current challenge. In Chapter 2, MXene (Ti2C) modified by 3-aminopropyl triethoxysilane was grafted onto the carbon fiber (CF) surface in an attempt to improve interfacial properties in continuous CF-reinforced epoxy composites. X-ray photoelectron spectroscopy, scanning electron microscopy, and dynamic contact angle test were employed to characterize the effect of the grafted Ti2C on the interfacial properties. A single fiber fragmentation test together with acoustic emission testing was performed to identify the interface failure mode and also determine the interfacial shear strength (IFSS). The interlaminar shear strength (ILSS) of the laminates was also evaluated with three-point beam testing. It was experimentally observed that Ti2C sheets were uniformly grafted on the fiber surface with covalent bonding. It could provide not only the increase of the CF surface roughness but also an excellent opportunity to create plenty of the polar functional groups thereby leading to a greater surface energy of the CF. The IFSS and ILSS of Ti2C modified CF composites were enhanced by ~78% and ~28% increase, respectively, compared to ones of unsized CF composites. This study shows the great potential of Ti2C as an excellent fiber sizing agent for manufacturing high-performance CF composites. In Chapter 3, a high-performance, printable lignin-based polylactic acid (PLA) composite was investigated through copolymerizing 2-ethylhexyl acrylate at the interface. It was shown that 10 wt% modified lignin (e-lignin) composites exhibit significantly enhanced toughness from 1.16 to 3.84 MJ/m3 and also impact energy from 2.12 to 6.36 KJ/m2 relative to the pure PLA. The responsible toughening effect was interpreted by the plasticization and the bridging effect of e-Lignin. The low melt viscosity of the dispersed e-Lignin phase causes local thermo-rheological relaxation and promotes the mobility of PLA molecular chains, showing desirable melt viscosity for fused deposition modeling 3D printing. Notable that the adhesion strength between deposited layers during the additive manufacturing was increased due to the high interfacial diffusion of composites, where an approximately 138% improvement of weld energy was achieved in 10 wt% e-lignin composites compared to those of pure PLA. This study shows the great promise to utilize lignin extracted natural materials, particularly in additive manufacturing by encouragingly replacing petroleum-based thermoplastics. In Chapter 4, a UV-curable lignin-based bio-hydrogel was produced via methacrylation using glycidyl methacrylate, followed by cross-linking with acrylamide. The synthesized lignin hydrogel showed outstanding compressive performance, durability properties, and rheological behaviors, which could be regulated by changing the lignin content. With increasing lignin concentration, the crosslinking density and storage/loss modulus increased, whereas tan δ decreased. Additionally, the hydrogels exhibited excellent swelling ratios (several thousand %), meanwhile, their swelling behavior and dimensional changes varied with the lignin concentration and strongly depended on temperature and pH. Finally, a smart strip-shaped hydrogel was designed to display actuator behavior, showing a fast and reversible swelling-deswelling response between pH 3 and 9. This hydrogel is a promising material for biomedical applications and 4D printing materials. In chapter 5, the above research topics were re-summarized.

Characterization of nanocarbon reinforced Al composite fabricated by liquid process

오세일 Graduate School, Korea University 2011 국내석사

Light metal matrix composites are of great interest for their role in reducing CO2 emissions through lightweight design. Nanocarbon/Al composites have recently emerged as promising materials because of their superior specific strength. Thus far, most nanocarbon/Al composites have been fabricated by powder metallurgy(PM) methods. Moreover, the referenced properties of the nanocarbon/metal composites are the result of these fabrication techniques. However, there is a limitation in scaling up. One of the fabrication methods overcoming this limitation is the liquid process; however, there are challenges to overcome in preparing nanocarbon/Al composites: (1) poor wetting of the nanocarbon with the molten aluminum, (2) entanglement of the nanocarbon owing to Van der Waals forces, and (3) floating of the nanocarbon on the melt surface owing to differences in specific gravity. To solve these problems, in this study, Cu coating on the nanocarbon was considered. In order to evaluate the wetting behavior of Al on Cu-coated graphite, wetting experiments using the sessile drop method were conducted. Results of these experiments showed that the Cu coating layer improved the wetting between Al and graphite. On the basis of this result, the nanocarbon was plated using Cu by the electroless plating method. The electroless Cu coating technique overcame the difference in specific gravity and prevented the entanglement due to Van der Waals forces and the formation of the brittle Al4C3 phase. The nanocarbon/Al composites showed significantly improved mechanical properties with increased nanocarbon content. The influence of the nanocarbon concentration on the composites was also studied.

Preparation of Conductive Epoxy Composite Using Bisphenol-Type Epoxy and Copper-Deposited Carbon Materials

임용찬 성균관대학교 일반대학원 2017 국내박사

Thermally conductive materials have received attention due to the miniaturization of electronic devices and development of aerospace technology. Thermal dissipation from inside of devices to outside is crucial technique for preventing problems such as overheating or even explosion. Polymer material can be excellent candidate for thermal issue in terms of basic property, process ability and economics etc. However polymer resins have decisive weakness to be used as conductive material, which is very low electrical and thermal conductivity. Therefore polymer usually applied to conductive material by being mixed with conductive filler such as nitride chemicals, metal, metal oxide and carbon-based materials. These materials are referred to as conductive polymer composite. Among a number of polymer resins, epoxy resin has outstanding mechanical, chemical and thermal properties. Especially bisphenol-type epoxy commonly has an advantage of loading filler because of flexible chemical structure. In this study, curing reaction of three kinds of bisphenol-type epoxy was analyzed using differential scanning calorimeter (DSC) kinetically. And three kinds of epoxy monomer and six kinds of curing agents were reviewed through measuring thermal conductivity using laser flash apparatus (LFA) method. Actually, thermal conductivity of epoxy composite is mainly under the control of conductivity of filler. However, thermal conductive path between filler and epoxy resin is also important because filler was surrounded by epoxy resins in composite. This is why many researchers have studied developing crystalline epoxy such as liquid crystalline epoxy resins. Meanwhile, copper nanoparticles were deposited on carbon black and graphite to increase thermal conductivity of carbon black and graphite which are very common conductive filler in diverse fields. Synthesis was examined by transmission electron microscope (SEM) and X-ray diffraction (XRD). The particles synthesized were mixed with epoxy selected above and the morphological, electrical, thermal and mechanical properties of epoxy composite were measured by SEM, 4-point probe, LFA and universal testing machine (UTM).

Carbon Composite과 Fe 이온 치환에 의한 LiFePO_(4)의 전기화학적 특성향상에 관한 연구

장수관 東新大學校 大學院 2004 국내석사

최근, LiFePO4는 환경친화적이며 원료가격이 낮고 열적 안정성이 우수하다는 장점으로 인해 많을 연구가 진행되어지고 있다. LiFePO4는 이런 좋은 전기화학적 특성이 있으나 전기 전도성과 이온전도성이 낮아 사용조건에 제한을 받는다. 본 연구에서는 이러한 낮은 전도성을 개선하고자 다음과 같은 carbon 복합체와 Fe치환에 의한 연구를 진행하였다. LiFePO4/C composite와 LiFe0.95M0.05PO4(M=Ca, Fe, Zn or Mg)는 용액법을 이용하여 합성되어졌다. 구조는 synchrotron X-ray diffraction, X-ray absorption spectroscopy와 Rietveld refinement를 이용하여 분석하였다. 합성분말의 입자 형태는 Scaning electron microscope, Transmission electron microscope에 의해 관찰하였다. 그리고 전기화학적 특성분석을 위하여 galvanostatic법으로 충방전 실험을 하였다. 첫째로, 우리는 하소온도와 carbon에 함량에 따른 LiFePO4/c composite의 영향을 연구하였다. LiFePO4에 합성은 550℃에서 850℃까지 Ar 분위기에서 실시되었고 carbon에 원료로 설탕을 첨가하여 조절하였다. 적정합성온도 이상에 하소와 과다한 설탕에 첨가는 전극물질에 volumetric energy density가 감소하였다. 최적에 합성조건은 750℃에 합성온도와 6.8wt%(vs. LiFePO4)에 carbon 함량으로 나타났다. 이런 적정 합성조건에서의 분말은 좋은 cycle-ability와 400mAg-1에 높은 전류밀도에서도 100mAhg-1이상에 우수한 rate-capability를 보였다. TEM 관찰에 의해, LiFePO4/C composite의 표면을 둘러싸고 입자사이를 이어주는 비정질 carbon이 형성되어 있는 것을 볼 수 있었다. 이러한 carbon 전도체는 전기전도성을 향상시키기 때문에 LiFePO4의 낮은 전기 전도성을 개선하기 위한 한 방법으로 사용하였다. 두 번째로, 용액법으로 합성되어진 LiFe0.95M0.05PO4/C (M=Ca2+, Fe2+, Zn2+ or Mg2+) composite는 약 300nm정도에 균일한 입자를 갖은 분말로 합성되었다. 격자상수와 단위체적은 M 이온의 이온반경이 작아짐에 따라 감소하였다. LiFe0.95M0.05PO4(M=Ca2+, Zn2+ or Mg2+)치환체의 전기화학특성을 시험한 결과 순수한 LiFePO4보다 낮은 분극현상을 보여 주었고 M 이온의 이온반경이 증가할수록 적은 용량감소 현상을 나타내었다. LiFePO4에서 Ca2+, Zn2+ 그리고 Mg2+ 치환은 전기전도의 제한을 감소시켰고 Ca2+ 치환은 전기화학적 특성을 향상시켰다. 그러나 이온반경이 작은 Zn2+나 Mg2+는 용량감소 경향을 보였다. Li0.05Fe3+0.95M2+0.05PO4의 전기화학적 특성은 치환체에 이온 반경에 영향을 받으며 치환은 특성향상을 위한 한 방법이다. 결론적으로, 본 연구에서는 전기전도성이 낮은 LiFePO4를 carbon 복합체와 2가의 전형원소의 치환으로 향상된 전기화학적 특성을 얻을 수 있었다. Recently, LiFePO4 have been extensively investigated by many researchers because it has some merits such as less toxic, inexpensive and good thermal property. Although LiFePO4 has good electrochemical properties, its utilization for cathode material in lithium secondary battery is limited by its low electronic conductivity. In this study, therefore, we tried to overcome this drawback through carbon composite and Fe substitution. LiFePO4/C composite and LiFe0.95M0.05PO4(M=Ca, Fe, Zn or Mg) were synthesized by solution method. Their structures were studied using synchrotron X-ray diffraction, X-ray absorption spectroscopy and Rietveld refinement. The particle morphologies of these materials were observed by Scaning electron microscope and Transmission electron microscope. The electrochemical properties were characterized using galvanostatic cycling. Firstly, we studied the effects of calcination temperature and carbon content on the electrochemical properties of LiFePO4/C composites. LiFePO4/C composites were calcined temperature from 550 to 850℃ in Ar atmosphere and carbon content in the composite was controlled by amount of table sugar that added as a carbon source. The high calcination temperature caused large grain growth and the large amount of sugar reduced volumetric energy density of cathode. The optimal conditions are calcination temperature 750℃ and carbon content 6.8wt%(vs. LiFePO4). The sample synthesized at the optimal conditions showed good cycle-ability and rate-capability as well as high discharge capacity over 100mAhg-1 at high current density 400mAg-1. From TEM observation, we found that amorphous nano-carbons in LiFePO4/C composite wrapped and connected LiFePO4 particles. We thought that nano-carbons in the composite improved electronic conductivity of the LiFePO4/C composite. Therefore, we suggest that carbon composite in this material is one of the ways to overcome low electronic conductivity of LiFePO4. Secondary, we synthesized LiFe0.95M0.05PO4/C (M=Ca2+, Fe2+, Zn2+ or Mg2+) composites whose average particle size is ca. 300nm by solution method. The lattice parameters and cell volume of the composite reduced with decreasing ionic radius of M ion. During electrochemical operation doped LiFe0.95M0.05PO4/C (M=Ca2+, Zn2+ or Mg2+) composites showed lower polarization than un-doped LiFePO4/C composite and the capacity loss decreased with increasing ionic radius of M ion. We found that the substitution of iso-valent ions such as Ca2+, Zn2+ and Mg2+ for Fe in LiFePO4 system reduced electronic limitation and electrochemical properties were improved by substitution of iso-valent Ca for Fe in LiFePO4 system. but the substitution of small ions, viz. Zn2+ and Mg2+, provokes capacity loss during electrochemical operation. Therefore, the electrochemical properties of LiFe0.95M0.05PO4/C (M=Ca2+, Fe2+, Zn2+ or Mg2+) composites are correlated with ionic radius of substituted ion and the substitution for Fe is another way to improve electronic conductivity of LiFePO4. Consequently, we conclude that low electronic conductivity of LiFePO4 can be improved through carbon composite and substitution for Fe.

Dispersibility and mechanical properties of polyetheretherketone/hydroxyapatite/carbon fiber composites prepared by non-melt blending method

전인성 서울대학교 대학원 2021 국내박사

Polyetheretherketone (PEEK), a member of the polyaryletherketone family, is a semi-crystalline thermoplastic polymer with combinations of ketone and ether functional groups. Due to its unique chemical structure, PEEK confers outstanding chemical resistance, mechanical properties, and biocompatibility. Thereby, PEEK has been a primary candidate to replace metallic implant components in the field of orthopedic surgery. However, the mechanical strength and the bioactivity of PEEK should be improved to be used as spinal implant component. To improve the mechanical strength and bioactivity of PEEK, PEEK is generally reinforced with carbon fiber (CF) and hydroxyapatite (HA) fillers. However, PEEK/HA/CF composites manufactured by the melt-extrusion process, commonly used in industry, are significantly inferior in processability due to the aggregation of nano-sized HA filler and de-bonding between the PEEK matrix and the fillers. Thus, other effective blending methods should be applied before the melt-extrusion process. In this study, two different fabrication methods (suspension blending and mechanofusion) were carried out to investigate the improvement in mechanical properties of PEEK/HA/CF composite. First, PEEK/HA/CF composite was prepared by suspension blending using surface modified HA and CF fillers. The surface modification was performed primarily with a silane coupling agent and secondly with succinic anhydride. The PEEK/HA/CF composite prepared in this way showed excellent improvement in both flexural and compressive strengths compared to the composite reinforced with unmodified fillers. The results of SEM and XRM analysis confirmed that both interfacial adhesion between the PEEK matrix and the fillers and dispersibility of HA were improved by surface modification on HA and CF fillers. Besides, HA nanofiber (HANF) was also used as reinforcement for PEEK composite. Due to its high elastic modulus and high aspect ratio, the PEEK/HANF/CF composite exhibited the highest mechanical strength, which was further enhanced after surface modification of fillers. These improvements were due to the enhanced dispersibility and interfacial adhesion of HANF and CF in the PEEK matrix, confirmed by SEM and XRM. Second, the PEEK/HA/CF composite was prepared by mechanofusion process, one of non-melt blending methods that can be performed in dry condition. The mechanofusion process is better method than the suspension blending method because it does not use any solvents. By mechanofusion process, the result of SEM confirmed that HA nanoparticles were uniformly coated on the surface of PEEK particles and micro-sized CF filler. This phenomenon could impede the formation of HA aggregates and helped the dispersion of HA filler during the injection molding process for PEEK/HA/CF composite. The PEEK/HA/CF composite prepared by mechanofusion method showed higher flexural and compressive strengths than the composite prepared by suspension blending method. The XRM analysis confirmed the enhanced dispersion of HA filler. Moreover, higher mechanical strength can be expected if the PEEK/HA/CF composite are prepared by the mechanofusion method using surface modified HA and CF. Finally, instead of using commercial PEEK, synthesized P(E2-E4)K polymer was used for the composite. In addition to CF, graphene oxide (GO), which has high surface area and various surface functional groups, had been introduced into P(E2-E4)K composite. GO was expected to increase the flexural strength with a small content compared to CF. But, the flexural strength was improved a little bit due to the aggregation of GO nanosheets. However, it was confirmed that the flexural strength of P(E2-E4)K/GO/CF composite is within the range of cortical bone if 30 wt% of CF was used. 폴리에테르에테르케톤 (polyetheretherketone, PEEK)는 폴리아릴에테르케톤 계열의 구성원으로 케톤과 에테르 작용기를 포함하는 반 결정성 열가고성 고분자이다. PEEK는 독특한 화학 구조로 인해 뛰어난 내화학성, 기계적 특성 그리고 생체 적합성을 가지고 있다. 따라서 PEEK는 정형외과 분야에서 금속 임플란트 소재를 대체 할 수 있는 주요 후보로서 사용되어 왔다. 하지만, 척추 임플란트 소재로 사용하기에는 기계적 강도와 생체활성 특성이 부족하여 주로 탄소섬유 (carbon fiber, CF)와 수산화인회석 (hydroxyapatite, HA) 충전재가 도입되어 사용된다. 그러나 산업에서 일반적으로 쓰이는 용융 압출 공정으로 PEEK/HA/CF 복합재를 제조할 시 열악한 가공성, HA 나노 입자의 뭉침 현상, 그리고 PEEK 기지재와 충전재간의 약한 계면 결합이 문제가 된다. 그렇기에 용융 압출 공정 외에 다른 효과적인 혼합 방식과 충전재 표면 개질 도입을 통해 위의 문제들을 해결할 필요가 있다. 첫째, HA와 CF를 표면개질을 하여 현탁 혼합 (suspension blending) 방식을 통해 PEEK/HA/CF 복합재를 제조하였다. 표면개질은 실란계 커플링제 (silane coupling agent)로 1차 그리고 석신산무수물 (succinic anhydride)로 2차 개질을 진행하였다. 이렇게 제조된 PEEK/HA/CF 복합재는 기존 표면 개질이 안된 충전재를 사용한 복합재에 비교하여 굴곡 및 압축 강도 모두에서 뛰어난 향상을 보였다. SEM과 XRM 분석을 활용한 형상학 분석을 통해 표면 개질된 충전재를 포함하는 복합재에서 PEEK 기지재와 충전재간의 계면접착이 크게 향상됨과 동시에 HA의 분산성이 향상 된 것을 확인하였다. 그리고 탄성률이 높은 HA 나노 섬유 (hydroxyapatite nanofiber, HANF) 를 도입하여 복합재에 큰 물성 강화 효과를 줄 수 있었으며, 동일한 표면 개질을 통해 PEEK/HANF/CF 복합재에서 가장 뛰어난 물성 향상을 확인하였다. 이러한 향상된 물성들은 PEEK 기지재 내에서 HANF와 CF의 향상된 분산성 및 계면접착력에 의한 것이며, 이는 SEM과 XRM 분석을 통해 확인되었다. 두 번째, 현탁 혼합 방식에는 많은 양의 용제가 쓰인다는 단점 있기에 용제가 사용되지 않는 비용융 혼합 방식인 메카노퓨전(mechanofusion) 방식으로 PEEK/HA/CF 복합재를 제조하였다. 메카노퓨전 방식을 통해 마이크로 크기인 PEEK 입자와 CF 충전재 표면에 나노 크기의 HA가 코팅이 된 것을 확인할 수 있었으며, 이는 PEEK/HA/CF 복합재 사출 성형 시 HA의 분산에 도움을 줄 수 있다. 이렇게 제조 된 복합재는 기존 현탁 혼합으로 제조된 복합재보다 더 높은 굴곡 및 압축 강도를 보였으며, XRM 분석을 통해 HA의 분산이 크게 향상되었음을 확인하였다. 또한, 표면 개질된 HA를 사용하여 메카노퓨전 방식으로 복합재를 제조하면 더 향상된 물성을 기대할 수 있다. 마지막으로, 상업용 PEEK가 아닌 합성된 P(E2-E4)K에 높은 표면적과 다양한 표면 작용기를 가지는 그래핀 옥사이드 (GO)를 도입하여 P(E2-E4)K/GO/CF 복합재를 현탁 혼합 방식으로 제조하였다. GO는 CF 대비 적은 함량으로도 높은 물성 향상을 기대하여 사용되었으나, 나노 충전재 특유의 뭉침 현상으로 인해 0.5wt% 이상 사용이 어려웠다. 하지만 CF 함량이 30wt% 포함될 시에는 척추 임플란트 소재로 쓰이기 적합한 수치의 굴곡 강도 수치를 보였다.